1. Field of the Invention
The present invention relates to a method for improving the morphology of refractory metal thin films by utilizing a low temperature phase transition. The improved method involves modification of the standard methods for the deposition of refractory metal films. The improved method includes both chemical vapor deposition (CVD) and physical deposition methods, such as sputter deposition.
2. Brief Description of the Prior Art
Refractory metals are character by their high melting points, exceeding an arbitrary value of 2000.degree. C., and low vapor pressures, two properties exploited by the electronics industry. Only four refractory metals, molybdenum, niobium, tantalum and tungsten, are available in quantities of industrial significance. Molybdenum, niobium, tantalum and tungsten have been produced commercially for many years, mainly as additives to steels, nickels and cobalt alloys and for certain electrical applications. In addition to high temperature strength, the relatively low thermal expansions and high thermal conductivities of the refractory metals suggest good resistance to thermal shock. There are however two characteristics, ready oxidation at high temperatures and, in the case of molybdenum and tungsten, brittleness at low temperatures, which limit their applications.
The metals to which the present invention applies comprises tantalum, niobium, molybdenum and tungsten.: Tantalum is a white lustrous metal resembling platinum. It is highly acid resistant and is classed as a noble metal. Tantalum will hold a fine cutting edge on tools. By adding silicon to tantalum, its hardness can be made close to diamond. Anode film made from tantalum has better dielectric properties than Al. It possesses a very low ductile-brittle transition temperature and a very versatile aqueous corrosion resistance. It can be effectively used in applications such as in electrodes in thermionic valves and capacitors. Corrosion resistant tantalum fabrications include thermocouple pockets, bayonet heaters, tube and sheet heat exchangers, electric lightbulb filaments, radio tubes operating at high temperatures, and current rectifiers.
Niobium also known as columbium is also called niobium possesses good corrosion resistance. It forms a superconducting alloy with tin. It has low neutron capture cross-section and very low ductile-brittle transition temperature, except when highly alloyed.
Molybdenum is typically used in electrodes in glass melting furnaces.
Tungsten is a heavy white metal which possesses the highest melting point of all metals. The metal is brittle and difficult to fabricate. Tungsten has a wide usage in alloy steels, magnets, heavy metals, electric contacts, light bulb filaments, rocket nozzles and especially electronic applications. Parts, rods, and sheet are made by powder metallurgy using tungsten powder of 99.99% purity, and rolling and forging are performed at high temperatures. The rolled metal and drawn wire have exceptionally high strength and hardness. Tungsten wire for spark plug and wire electronic use is made by powder metallurgy. Tungsten whiskers are used in copper alloys to provide strength. The properties of tungsten make it especially important in electronic use.
Refractory metals are widely used for the fabrication of microelectronic circuitry where their ability to withstand high temples is required. Tungsten in particular is widely used in various wiring applications. Refractory metals ,may also be used as precursors to the formation of suicides for various applications.
Other objects and features as well as additional details of the present invention will become apparent from the following detailed description and annexed drawings of the presently preferred embodiments thereof, when considered in conjunction with the associated drawings.